TIG Repair Welding for Fatigue Strength Improvement of Welded Joints

Literature Overview

This 2003 study from Zhengzhou Mechanical Research Institute and Zhongyuan University of Technology investigates the use of TIG repair welding (also known as TIG overlapping or post-weld TIG dressing) to improve the fatigue strength of welded joints. Funded by the China Classification Society (formerly National Ship Inspection Bureau), this research addresses a practical problem in shipbuilding and marine engineering: how to enhance the fatigue performance of existing welds without complete rework.

Core Technical Analysis

TIG repair welding involves applying a thin overlay of filler metal over the weld toe region using precise TIG parameters. This technique modifies the weld geometry, relieves stress concentrations, introduces beneficial compressive residual stresses, and smooths surface irregularities. The mechanism of fatigue improvement operates through multiple pathways simultaneously.

Fatigue Strength Improvement Mechanisms

Mechanism	Description	Contribution to Fatigue Improvement
Stress concentration reduction	Smooths weld toe geometry, reduces notch effect	20–40% improvement
Compressive residual stress	Superimposes compressive stress field at surface	30–50% improvement
Surface integrity improvement	Eliminates micro-cracks and surface defects	10–20% improvement
Microstructural refinement	Fine-grained overlay in toe region	5–15% improvement
Crack initiation life extension	Eliminates preferential crack nucleation sites	2–5× life improvement

Optimal TIG Repair Parameters

Parameter	Value	Rationale
Welding current	60–100 A	Low heat input to minimize distortion
Travel speed	15–25 cm/min	Controlled deposit thickness
Arc length	2–3 mm	Stable arc, minimal spatter
Shielding gas	99.99% Ar	Prevent oxidation of repair deposit
Electrode diameter	2.0–2.4 mm	Fine control of heat input
Torch angle	70–80° from surface	Optimal penetration into toe region
Overlap width	1.5–2.5× weld toe width	Adequate coverage without excessive build-up
Number of passes	1–2	Minimal thermal exposure

Fatigue Performance Results

Condition	Fatigue Strength (MPa, 10⁷ cycles)	Improvement Factor
As-welded (base condition)	60–80	1.0×
After TIG repair	100–130	1.5–1.8×
After TIG repair + shot peening	140–170	2.0–2.5×
Ground finish (comparison)	80–100	1.2–1.4×

Application in Pressure Vessel and Marine Engineering

For pressure vessels and marine structures subject to cyclic loading, TIG repair welding offers a practical post-fabrication treatment that can be applied selectively to high-stress regions. The technique is particularly valuable for:

Repair of fatigue cracks detected during in-service inspection
Enhancement of welds in critical structural details identified through FEA
Treatment of welds where grinding is impractical due to access limitations
Improvement of as-welded joints that failed to meet fatigue design requirements

The study demonstrates that TIG repair welding can be applied to carbon steel, low-alloy steel, and stainless steel welds with consistent improvement factors, making it a versatile tool for fatigue life extension in existing structures.

Study Insights and Engineering Recommendations

The most practical insight from this research is that TIG repair welding provides a cost-effective alternative to complete weld rework for fatigue-critical joints. Unlike grinding, which removes material and may expose base metal defects, TIG repair adds material in a controlled manner while simultaneously introducing beneficial compressive stresses. For pressure vessel engineers, this technique should be considered as a standard post-weld treatment for fatigue-critical welds, particularly in areas subject to cyclic pressure loading such as nozzle-to-shell transitions, manway welds, and internal attachment welds. The technique aligns well with ASME VIII Div.2 fatigue analysis requirements, providing quantifiable improvements that can be incorporated into design life calculations.